Most existing Wireless Local Area Network (WLAN) systems based upon OFDM modulation comply with either the IEEE 802.11a or IEEE 802.11g standards (hereinafter “IEEE 802.11a/g”). See, e.g., IEEE Std 802.11a-1999, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification: High-Speed Physical Layer in the Five GHz Band,” incorporated by reference herein. In order to support evolving applications, such as multiple high-definition television channels, WLAN systems must be able to support ever increasing data rates. Accordingly, next generation WLAN systems should provide increased robustness and capacity.
Multiple transmit and receive antennas have been proposed to provide both increased robustness and capacity. The increased robustness can be achieved through techniques that exploit the spatial diversity and additional gain introduced in a system with multiple antennas. The increased capacity can be achieved in multipath fading environments with bandwidth efficient Multiple Input Multiple Output (MIMO) techniques.
A MlMO-OFDM system transmits separate data streams on multiple transmit antennas, and each receiver receives a combination of these data streams on multiple receive antennas. The difficulty, however, is in distinguishing between and properly receiving the different data streams at the receiver. A variety of MIMO-OFDM decoding techniques are known, but they generally rely on the availability of accurate channel estimations. For a detailed discussion of MIMO-OFDM decoding techniques, see, for example, P. W. Wolniansky at al., “V-Blast: An Architecture for Realizing Very High Data Rates Over the Rich-Scattering Wireless Channel,” 1998 URSI International Symposium on Signals, Systems, and Electronics (September 1998), incorporated by reference herein.
In order to properly receive the different data streams, MIMO-OFDM receivers must acquire a channel matrix through training. This is generally achieved by using a specific training symbol, or preamble, to perform synchronization and channel estimation techniques. The training symbol increases the total overhead of the system. In addition, a MIMO-OFDM system needs to estimate a total of NtNr channel elements, where Nt is the number of transmitters and Nr is the number of receivers, which could lead to an Nt increase of the long training length.
A need therefore exists for a method and system for performing channel estimation and training in a MIMO-OFDM system utilizing a signal that is orthogonal in either the frequency domain or the time domain. A further need exists for a method and system for performing channel estimation and training in a MIMO-OFDM system that is compatible with current IEEE 802.11a/g standard (SISO) systems, allowing MIMO-OFDM based WLAN systems to efficiently co-exist with SISO systems.